Support for external PRs

[lemaitre-aneo]

The events push and pull_request are not compatible with external pull requests. The former is never triggered, and the latter has no secrets when coming from an external repo, and thus cannot be published.

The best way to move forward is likely to build and test on pr.synchronize, and publish on pr.merged

[lemaitre-aneo]

https://stackoverflow.com/questions/74957218/what-is-the-difference-between-pull-request-and-pull-request-target-event-in-git

[esoubiran-aneo]

https://docs.github.com/en/actions/learn-github-actions/contexts#github-context